The Universal Mobile Telecommunication System (UMTS) is one of the third generation mobile communication technologies designed to succeed the Global System for Mobile communication (GSM). 3GPP Long Term Evolution (LTE) is a project within the 3rd Generation Partnership Project (3GPP) to improve the UMTS standard to cope with future requirements in terms of improved services such as higher data rates, improved efficiency, lowered costs etc. The Universal Terrestrial Radio Access Network (UTRAN) is the radio access network of a UMTS and Evolved UTRAN (E-UTRAN) is the radio access network of an LTE system. In an E-UTRAN, a user equipment (UE) 150 is wirelessly connected to a radio base station (RBS) 110a commonly referred to as an eNodeB or eNB (E-UTRAN NodeB), as illustrated in FIG. 1a. The eNBs 110a-c are directly connected to the core network (CN) 190.
In a wireless communication system using Orthogonal Frequency Division Multiplexing (OFDM) technology, the entire channel is divided into many narrow sub-channels, which are transmitted in parallel. This technique thus transforms a frequency selective wide-band channel into a group of non-selective narrowband channels, making it robust against large delay spread by preserving the orthogonality in the frequency domain. The primary advantage of OFDM over single-carrier schemes is its ability to cope with severe channel conditions without complex equalization filters in the receiver. Channel equalization is simplified because OFDM may be viewed as using many slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. The low symbol rate makes the use of a guard interval between symbols affordable, making it possible to handle time-spreading and eliminate inter-symbol interference.
In an LTE system the OFDM technology is adopted as a mean to achieve high transmission capability and robustness to multi-path delay. Orthogonal Frequency-Division Multiple Access (OFDMA) is used in the downlink, and Single-carrier frequency-division multiple access (SC-FDMA) is used in the uplink. OFDMA is a multi-user version of OFDM, where multiple access is achieved by assigning subsets of subcarriers to individual users. SC-FDMA is a linearly pre-coded OFDMA scheme. The distinguishing feature of SC-FDMA is that it leads to a single-carrier transmit signal, in contrast to OFDMA which is a multi-carrier transmission scheme.
Channel estimation has been widely used to improve the performance of OFDM systems, and is crucial for e.g. diversity combination, coherent detection, and space-time coding. Pilot based channel estimation schemes are commonly used. In such a scheme known reference symbols—also called pilot symbols—are inserted at regular intervals in the time-frequency grid. Using the knowledge of the pilot symbols, the receiver can estimate the frequency domain channel around the location of the pilot symbol. The channel's frequency response across the whole bandwidth can thus be determined by interpolation using various channel estimation schemes, such as Least Square (LS), Linear Minimum Mean Square Error (LMMSE), or Discrete Fourier Transform (DFT) based estimation. With the DFT based estimation, the frequency domain channel estimate is transformed into a time domain channel estimate, and the time domain properties of the channel are used instead of the frequency domain properties when estimating the channel.
In OFDM systems such as LTE, where OFDMA and SC-FDMA are used for downlink and uplink respectively, each user only accesses a part of the available frequency bandwidth. This means that channel estimates are only available for a part of the complete frequency bandwidth, and therefore the time domain channel estimate cannot be obtained from the conventional DFT based estimation scheme. The reason is that an inverse discrete Fourier transformation (IDFT) of a partial frequency response, leads to a channel impulse response where channel power leaks to all taps in the time domain due to sampling at non-integer tap positions. The unwanted noise and the useful channel power that has leaked out in the time domain will therefore be mixed up, and the conventional DFT based estimation scheme will not only eliminate the noise but will also eliminate some of the useful leakage power as they cannot be distinguished from each other.